Ashless nitrogen and ester containing lubricating oil dispersants have been widely used by the industry. Typically, these dispersants are prepared from a long chain hydrocarbon polymer by reacting the polymer with maleic anhydride to form the corresponding polymer which is substituted with succinic anhydride groups. Polyisobutylene has been widely used as the polymer of choice, chiefly because it is readily available by cationic polymerization from butene streams (e.g., using AlCl.sub.3 catalysts). Such polyisobutylenes generally contain residual unsaturation in amounts of about one ethylenic double bond per polymer chain, positioned along the chain.
The polyisobutylene polymers (PIB) employed in most conventional dispersants are based on a hydrocarbon chain of a number average molecular weight (M.sub.n) of from about 900 to about 2500. PIB having a M.sub.n of less than about 300 gives rather poor performance results when employed in dispersants because the molecular weight is insufficient to keep the dispersant molecule fully solubilized in lubricating oils. On the other hand, high molecular weight PIB (M.sub.n &gt;3000) becomes so viscous that conventional industrial practices are incapable of handling this product in many operations. This problem becomes much more severe as the PIB molecular weight increases to 5000 or 10,000.
Increased amounts of terminal ethylenic unsaturation in polyisobutylene (so-called "reactive polyisobutylene") has been achieved by BF.sub.3 catalyzed polymerization of isobutylene. Exemplary of references disclosing these polymers is U.S. Pat. No. 4,152,499. However, such reactive polyisobutylene materials can still contain substantial amounts of unsaturation elsewhere along the chain. Further, it is difficult to produce such reactive polyisobutylene polymers at molecular weights of greater than about 2,000, and, even so, the reactive polyisobutylenes themselves still suffer the above-noted viscosity increase disadvantages as molecular weights are increased.
Other polymers, such as ethylene-propylene co-polymers and terpolymers containing non-conjugated dienes, have been disclosed as suitable polymers for the preparation of ashless nitrogen and ester dispersants.
U.S. Pat. No. 4,234,435, for example, discloses dispersants prepared from polyalkenes, M.sub.n of 1,300 to about 5,000. The polyalkene can comprise homopolymers or interpolymers of C.sub.2 to C.sub.16 terminal olefins, of which ethylene-propylene copolymers are said to be examples, with specific reference to a copolymer of 804 ethylene and 20% propylene.
However, ethylene-alpha-olefin copolymers of the above molecular weights could be produced using Ziegler-Natta catalysts only in combination with H.sub.2 as molecular weight control in order to terminate the growing copolymer chains within this molecular weight range. Without use of H.sub.2 or other conventional, so-called "chain stoppers", the copolymers produced with Ziegler-Natta catalysts would tend to have molecular weights greatly in excess of the above range. (Such higher copolymers, for example, are widely employed in ungrafted form as viscosity index improvers, and when grafted with nitrogen-containing groups, as described below, are conventionally employed as dispersant-viscosity index improver polymers.) The use of H.sub.2 as a chain stopper has the disadvantage of causing the saturation of the olefinic double bond content of the copolymer. Thus, while lower molecular weight copolymers were theoretically possible to prepare, their low unsaturation content (and the accompanying low graft copolymer yields) would have made their further functionalization by a thermal "enell reaction, e.g., with dicarboxylic acid moieties in preparing dispersants, highly unattractive.
U.S. Pat. No. 4,668,834 to Uniroyal Chemical discloses preparation (via certain metallocene and alumoxane catalyst systems) and composition of ethylene-alpha olefin copolymers and terpolymers having vinylidene-type terminal unsaturation, which are disclosed to be useful as intermediates in epoxy-grafted encapsulation compositions.
U.S. Pat. No. 4,704,491 to Mitsui Petrochemical relates to liquid ethylene alpha-olefin random copolymers, useful when hydrogenated as synthetic lubricant oil, characterized inter alia by having 10-85 mol.% ethylene units, 15-90 mol.% alpha-olefin units, M.sub.n of from 300 to 10,000 and a w/M.sub.n of not more than 2.5. The patent also indicates that the liquid copolymer can be easily modified since it has a double bond capable of reacting with maleic anhydride, etc., at the molecular chain ends.
Japanese Published Patent Application 87-129,303A of Mitsui Petrochemical relates to narrow molecular weight distribution (M.sub.w /M.sub.n &lt;2.5) ethylene alpha-olefin copolymers containing 85-99 mol% ethylene, which are disclosed to be used for dispersing agents, modifiers or materials to produce toners. The copolymers (having crystallinity of from 5-85%) are prepared in the presence of a catalyst system comprising Zr compounds having at least one cycloalkadienyl group and alumoxane.
European Patent 128,046 discloses (co)polyolefin reactor blends of polyethylene and ethylene higher alpha-olefin copolymers prepared by employing described dual-metallocene/alumoxane catalyst systems.
European Patent Publication 129,368 discloses metallocene/alumoxane catalysts useful for the preparation of ethylene homopolymer and ethylene higher alpha-olefin copolymers.
European Patent Application Publication 257,696 Al relates to a process for dimerizing alpha-olefins using a catalyst comprising certain metallocene/alumoxane systems.
European Patent Publication 305,022-Al to Mitsui Petrochemical relates to certain synthetic hydrocarbon lubricating oil compositions containing a load-withstanding additive and a liquid ethylene alpha-olefin random copolymer modified by graft copolymerization with an unsaturated carboxylic acid or derivative thereof (e.g., maleic anhydride) . The ethylene alpha-olefin copolymers (M.sub.n of 300 to 12,000) are obtained using Ziegler catalysts (e.g., catalyst formed from soluble V compound and an organo aluminum compound), are grafted in the presence of a free radical initiator.
PCT Published Patent Application WO 88/01626 relates to transition metal compound/alumoxane catalysts for polymerizing alpha-olefins.